1. Field of the Invention
The present invention relates to an ion plating apparatus which forms a film on a substrate by ionizing the vapors of evaporating materials, accelerating the ionized particles of the vapors by an electric field and making them impinge on the substrate.
2. Description of the Prior Art
Vacuum deposition, sputtering, ion plating and the like have been heretofore known as techniques to form a film on the surface of a substrate of metal, insulating material or semiconductor material under vacuum.
Among these techniques, ion plating is a technique in which the vapors of evaporating materials to be deposited on a substrate are fed into a plasma or bombarded with electrons thereby to be ionized, and the ionized particles of the above-mentioned vapors are accelerated by an electric field, thereby to impinge on the substrate with high energy.
Accordingly, in the ion plating method, the high-energy ionized particles are deposited on the substrate while sputtering the surface of the substrate. Therefore, the surface of the substrate is at all times kept clean, and consequently the film produced thereon are strong, high in adhesion and of close texture almost free of pinholes.
In the ion plating method also, the formation of films are usually performed in a high-vacuum atmosphere having a pressure of 10.sup.-2 Torr or less. The films thus obtained are high in quality, since they are hardly influenced by materials except the evaporating materials. In addition, the deposition rate is greater than that in the case of the sputtering method. Especially, when the above-mentioned ionization is performed by electron bombardment under a high vacuum having a pressure of 10.sup.-4 Torr or less, the films produced become more close in texture and high in crystallinity. In this case, acceleration voltage may be lowered to prevent increase of the substrate temperature.
For the purpose of carrying out the ion plating method, there have been proposed various types of apparatuses according to the modes of ionization of evaporating materials. FIG. 1 shows the schematical construction of an example of these apparatuses.
In FIG. 1, reference numeral 1 designates a boat which contains evaporating materials 2 to be deposited on the substrate surface. The evaporating materials 2 placed in the boat 1 are heated and vaporized by a suitable heating means, for instance, resistance heating or electron beam heating.
The evaporating materials 2 thus vaporized are fed into an ionization chamber 3 where they are ionized. The ionization chamber 3 is composed of a filament 4 and shield boards 5 for isolating the filament 4 from other regions. The filament 4 is in the form of, for instance, a coil, being energized by a power supply 12 so that it is heated to emit thermions. The thermions thus emitted from the filament 4 are accelerated toward the boat 1 by a power supply 11 provided between the filament 4 and the boat 1. While moving toward the boat 1, the thermions impinge on the vapor particles of the evaporating materials 2 and thereby ionize them.
Reference numeral 6 designates a shutter for preventing as necessary the ionized particles of the evaporating materials 2 from impinging on the substrate surface. Reference numeral 7 designates a substrate holder for holding a substrate 8. The substrate holder 7 is kept at a high negative potential with respect to the ionization chamber 3 by a power supply 13 and thereby serves also as an electrode for giving kinetic energy to the above-mentioned ionized particles so as to move them in the direction of the substrate 8. The ionized particles of the evaporating materials 2 impinge on the surface of the substrate 8 at high speed and with high energy to form a film 9 thereon.
Reference numeral 10 designates a heater for heating the substrate 8. If, for instance, the substrate 8 is assumed to be monocrystalline and the evaporating materials 2 are to be epitaxially grown on this monocrystalline substrate 8, the heater 10 is used to heat the substrate 8 to a temperature required for epitaxial growth, or epitaxial temperature.
Except the power supplies 11 to 14, all the parts and regions shown in FIG. 1 are usually kept at a high vacuum having a pressure of 10.sup.-2 or less by provision of, for instance, a vacuum vessel not shown.
As mentioned above, the film 9 formed by ion plating is very high in adhesion to the substrate 8, close in texture and high in quality. Recently, ion plating has attracted increasing attention from various fields not only as a technique for forming coatings on the surfaces of metallic or insulating materials but also as a technique for forming the thin-film elements of semiconductors, dielectrics and magnetic materials. In addition, various efforts are being made to improve the ion plating method and also the apparatus therefore.
For instance, the following methods have been proposed in order to efficiently ionize the evaporating materials:
One is the method in which a current-collecting anode is provided at the center of the coil-shaped filament so as to accelerate and collect thermions emitted from the filament.
The other is the method in which a cylindrical mesh-shaped electron-accelerating grid is provided inside the coil-shaped filament concentrically therewith so that the thermions emitted from the filament are accelerated between the filament and the grid and enter the inside of the grid through the mesh thereof to ionize the evaporating materials.
Generally, in the ion plating method, each atom or molecule is ionized, being accelerated by an electric field to impinge on the substrate surface thereby forming a film thereon. Recently, however, a new version of ion plating called the cluster ion beam deposition method has been proposed and put into practical use. In the cluster ion beam deposition method, evaporating materials are heated and vaporized in a closed type crucible having at least one nozzle. The vapors of the evaporating materials are jetted from the nozzle into a high vacuum region where a number of atom groups each consisting of about 100 to 2,000 atoms and loosely bonded together by van der Waals forces are formed under the influence of a supercooling phenomenon caused by the adiabatic expansion of the vapors at the time when they are jetted. The above-mentioned atom groups, or clusters, are formed into cluster ions by ionizing at least one of the atoms thereof, and the cluster ions are accelerated by an electric field to impinge on the surface of the substrate thereby forming a film thereon.
In addition to the features of the usual ion plating method, the cluster ion beam deposition method has the following excellent features:
When the above-mentioned cluster ions impinge on the substrate, they are disintegrated into atomic particles. These atomic particles roll on the substrate surface thereby contributing to film formation. This phenomenon is usually called the surface migration effect. Because of this surface migration effect, the cluster ion beam deposition method can produce a film high in crystallinity and also high in adhesion to the substrate and between its atoms. In addition, the cluster ion beam deposition method can effectively form a film on a substrate of insulating material because the charge-mass ratio e/m is small.
As mentioned above, various studies and improvements have been made on the apparatus and method associated with the ion plating technique, and thereby various thin-film elements have become able to be produced. In this connection, a method of producing thin films of compounds will be hereinafter described by way of example.
FIG. 2 schematically shows the arrangement of the vapor sources and ionization chamber of the apparatus for producing two-element compounds by the above-mentioned cluster ion beam deposition method.
In FIG. 2, reference numerals 21 and 22 designate closed type crucibles having nozzles 21a and 22a, respectively. Evaporating materials 23 and 24 corresponding to the component elements of a compound to be produced are placed in the crucibles 21 and 22, respectively. These evaporating materials 23 and 24 are heated by heaters (not shown) and thereby their vapors are produced. The crucibles 21 and 22 are held in a high-vacuum atmosphere having a pressure of at least 1/100 or less of the pressure of the vapors in the crucible 21 or 22. The evaporating materials 23 and 24 vaporized are jetted from the nozzles 21a and 22a, respectively, and are converted into clusters C.sub.1 and C.sub.2 under the influence of supercooling caused by the adiabatic expansion of the vapors, respectively.
Ionization chambers 29 and 30 are provided in the vicinity of the nozzles 21a and 22a, respectively. These ionization chambers 29 and 30 are composed of filaments 25 and 26 adapted to emit thermions when energized and heated and shield boards 27 and 28, respectively. Power supplies for ionization are provided between the ionization chamber 29 and the crucible 21 and between the ionization chamber 30 and the crucible 22. Thus, electrons emitted from the filaments 25 and 26 are accelerated. At least one of the atoms of each of the clusters C.sub.1 and C.sub.2 of the evaporating materials 23 and 24 entering the ionization chambers 29 and 30 are ionized by the above-mentioned accelerated electrons to form cluster ions C.sub.3 and C.sub.4, respectively.
The cluster ions C.sub.3 and C.sub.4 are accelerated by electric fields for acceleration, impinging on the substrate surface with high energy thereby to form the film of a compound.
In producing compounds by the ion plating method such as the above-mentioned cluster ion beam deposition method, the component elements of the compound are different in ionization voltage from one another and therefore it has been a usual practice to provide ionization chambers, one for each component element of the compound.
Accordingly, in producing a multi-element compound, it becomes necessary to provide ionization chambers the number of which corresponds to that of the component elements of the compound. If, for instance, an impurity, or dopant is to be added to the compound, it becomes necessary to provide another ionization chamber for this impurity. As a result, the apparatus for this method inevitably becomes large and complicated.
In producing semiconductors or magnetic materials by the ion plating method, it is well known that the ionization rate of the evaporating materials has great influence on the crystallinity and thickness of the film.
For instance, if the ionization chamber is cylindrical as shown in FIG. 1 or FIG. 2, the density of ionization electron current at the center of the ionization chamber differs from that at the periphery thereof, and therefore the ionization rate of the evaporating materials at the center of the ionization chamber differs from that at the periphery thereof.
Accordingly, the sputtering effect produced by the ionized evaporating materials on the substrate surface becomes nonuniform resulting in uneven film thickness, uneven adhesion, uneven crystallinity, etc. Therefore, it is difficult to produce a uniform film high in crystallinity by the use of the conventional ion plating method.